Our trip to CAT Driver Training at Millbrook revealed that the car will lock its rear wheels before the front wheels. This is not good and needs fixing. we had to cut our driver training day short, but CAT have been very good about it. Some more information here (blog) and here (Pistonheads). On a handling pan at Millbrook the on-limit grip during cornering seemed ok and Colin liked the way the car handled.
We had TVR Power look at the brakes (and a drip from the radiator) on the way back from Millbrook. A quick visual inspection had shown that the rear pads were EBC Greenstuff, but the front pads were not visible. Jason at TVR Power suggested changing all front and back to AP Racing.
A quick bit of research suggested that EBC Greenstuff may have a much higher coefficient of friction than the AP Racing pads. The EBC site suggests a coefficient of 0.55, whereas the AP Racing site suggests more like 0.44 for their pads when warm, and less when cold. This would mean the rears could have been braking with 37.5% more force than designed if the fronts were cold, and that might well be enough to cause the problem.
I am not sure why the AP pads gave a lower coefficient of friction, but it would be interesting to know the temperature response characteristics of the EBC pads. They may work less well when very hot.
When the car was in for a new radiator and rear pads, the front brake pads were examined. The fronts were shown to be the correct AP pads (must be DS2500 by a process of elimination, but I haven’t checked) so were left in place and the rears were changed to AP (DS25HP, a low-noise variant of the DS2500).
Dom then took the car for a good test drive, and said the brake balance seemed fine. However, when I got round to testing it, the balance was still off and the rears locked first on dry tarmac. I think the driver’s side rear was locking, not sure about passenger side. If they both locked it would stall the car which I am sure I would notice.
Next up was a 3000 mile European road trip. This was fine – the brake problem only manifests itself right at the limit.
Following the road trip the car was due its annual service at Track v Road, the garage that sold us the car.
They suggested fitting an AP Racing adjustable brake bias valve in the rear brake lines. They checked the front brakes and thought they were fine.
A write-up of the service, with some pictures of the valve installation, is here.
I had hoped the valve would fix the problem. Testing is not yet complete as it is hard work to do on-limit brake tests. This isn’t helped by the fact that the valve rattles when it gets towards the end of its adjustment, and the end when it should do the most to fix the bias issue. See this writeup for details of the testing and results. See this thread on Pistonheads for some discussion of the problems.
What to try next?
The problem is not yet fixed. It may be that the final bit of adjustment in the valve would fix it, but then it makes a very annoying rattly noise when driven on anything but the smoothest surface. Also it is coming into winter and becoming harder to test.
I have been in communication with Ford to find out the specification of the standard brake proportioning valve which is fitted – Ford part 7117274. The initial response from Ford was not helpful but after some further communications it seems that they will send me the specification. This will let me know where in the AP valve’s range of adjustability the standard valve sits.
Changing the front pads for new pads may help. I don’t know the history of the fronts. I have been told they are the correct model and that they have plenty of pad material left, but age is unknown and it is possible that they could be past their best. The knowledge I have of the brake pads is here.
The master cylinder could be faulty. I don’t imagine that would be easy to test so wold probably involve a straightforward swap of old for new at £120 for the cylinder and a fair bit for labour.
Car suspension setup is also an option. It has been looked at by good TVR garages, should be set-up correctly, and handles fine (to me), but maybe there is something that is missing. I could have it looked at by somewhere like Centre Gravity who have a very good reputation.